A propulsion unit comprises a strut having an upper portion and a lower portion. An upper end portion of the upper portion passes through a passage formed between a first outer bottom and a second inner bottom of a vessel. The upper end portion is rotatable supported at the hull of the vessel with a slewing bearing and sealed against the hull of the vessel with a slewing seal. The slewing seal prevents lubrication medium leakage from the slewing bearing to the sea. The slewing seal prevents further water from penetrating into the vessel.
The slewing bearing is in one prior art solution positioned below the second inner bottom of the vessel in the space between the first outer bottom and the second inner bottom of the vessel. The slewing seal is positioned immediately below the slewing bearing. The slewing bearing comprises an upper slewing seal preventing lubrication medium leakage from the slewing bearing to the sea and a lower slewing seal preventing sea water from penetrating into the vessel and into the slewing bearing. The upper slewing seal and the lower slewing seal are formed as one entity in a common recess. The slewing seal is positioned so that it is only accessible from above. This means that the slewing bearing and the gearwheel positioned on the slewing bearing has to be disconnected and removed before access to the slewing seal is provided.
It is thus rather difficult and time consuming to service and change the slewing seal in such prior art slewing seal arrangements. The condition of the slewing seal has been monitored indirectly by monitoring the condition of the lubrication medium. Water in the lubrication medium indicates that the slewing seal is leaking. Visual inspection of the slewing seal without dismantling the slewing bearing and the gearwheel is only possible e.g. through bores penetrating into the area of the slewing seal. This means that it is rather difficult to monitor the condition of the slewing seal. The lifetime of the slewing seal is normally shorter than the lifetime of the slewing bearing. The slewing seal should thus be changed more often than the slewing bearing. There is thus a risk that the slewing seal will not be changed as often as would be optimal.